This invention relates to cast composite material, and, more particularly, to the preparation of such cast composite materials having an Al-Mg matrix and a reinforcing particulate such as aluminum oxide that is reactive with magnesium.
Cast composite materials are conventionally formed by melting a matrix alloy in a reactor and then adding short, discontinuous particles. The mixture is vigorously mixed to encourage wetting of the matrix alloy to the particles, and after a suitable mixing time the mixture is cast into molds or forms. The mixing is conducted while minimizing the introduction of gas into the mixture. The resulting composite materials have the particulate reinforcement distributed throughout a matrix of an alloy composition.
Such cast composite materials are much less expensive to prepare than other types of metal-matrix composite materials such as those produced by powder metallurgical technology and infiltration techniques. Composite materials produced by this approach, as described in U.S. Pat. Nos. 4,759,995, 4,786,467, and 5,028,392, have enjoyed commercial success in only a few years after their first introduction.
Desirably, the cast composite materials have fully wetted particles, few voids, and a generally uniform microstructure. Complete wetting is necessary to realize the full composite strength and other mechanical properties. Equally important is the need to avoid the formation of deleterious phases that may adversely affect the microstructure and the mechanical properties of the finished cast composite material.
The presence of magnesium in the aluminum-alloy matrix of cast composite materials reinforced with aluminum oxide particulate has posed a significant problem. Magnesium on the order of 1/2 percent of more is required in many aluminum alloys to achieve their full strengths during aging treatments. Aluminum matrix alloys with such large amounts of magnesium, on the order of 1/2 percent or more of the matrix, readily wet aluminum oxide particulate, but may also react with the particulate to produce the brittle spinel phase, MgAl.sub.2 O.sub.4. The formation of the spinel phase is the principal cause of a reduction in matrix alloy magnesium content, which in turn prevents the matrix alloy from reaching its full strength potential during subsequent aging treatments. The amount of spinel formed is dependent upon three factors: the magnesium content of the matrix alloy, the mixing temperature, and the mixing time. Under normal mixing conditions, where the mixing temperature is 680-730 C. and the mixing time is 1-2 hours, the magnesium content of the alloy matrix becomes the principal determining factor of the amount of spinel formed. Aluminum matrix alloys with small amounts of magnesium do not exhibit extensive spinel formation, but also do not readily wet the aluminum oxide particulate.
There are a number of techniques that can be applied to enhance wetting or control chemical interactions between the matrix and the particles, which may work in some circumstances. The particles can be modified with special coatings, but the coating operation can significantly raise the cost of the particles and the composite material. Small amounts of reactive gases can be introduced into the mixing chamber, but the improved wetting may only be achieved at the cost of increased porosity in the cast composite material. Another approach to improved wetting is to raise the temperature at which the mixing is accomplished, but increased temperature also results in the acceleration of the production of deleterious phases where such phases are thermodynamically favored but kinetically slow in forming at lower temperatures.
There therefore exists a continuing need for an improved technique for producing cast composite materials of aluminum-magnesium alloys and reactive particles, especially aluminum oxide particles. The present invention fulfills this need, and further provides related advantages.